Electric resistance heater

ABSTRACT

Small electric resistance elements for temperatures in the range of up to 1,680* C and made of at least 70 percent by volume MoSi2 and up to 30 percent by volume of an addition having grain growth inhibiting properties at high temperatures, the particle size in the body of the finally manufactured element being less than onefifteenth of the cross dimension of the element in the incandescent zone thereof and, in any case, less than 20 microns and further provided with a dense superficial layer of SiO2 glass the thickness of which is between 1 and 8 microns. The incandescent zone suitably has a rectangular cross section having specified dimensions. The grain growth inhibiting addition may suitably be silicon carbide or a glass.

United States Patent Magnusson et a].

[ 1 Aug. 1,1972

[541 ELECTRIC RESISTANCE HEATER 22 Filed: May 18,1970

21 App1.No.: 38,416

[30] Foreign Application Priority Data May 16, 1969 Great Britain ..25,73l/69 May 16, 1969 Great Britain ..25,230/69 [52] US. Cl. ..338/262, 29/191 .6, 2l9/553, 338/330, 338/333, 117/201 [51] Int. Cl. ..l-l0lc 1/00 [58] Field oi'Searcli ..ll7/20l; 338/333,334, 262, 338/330; 252/516; 219/553; 29/l91.6

[56] References Cited UNITED STATES PATENTS 254,032 2/1882 Maxim ..338/330 3,321,727 5/1967 Schrewelius ..338/330 Primary Examiner-E. A. Goldberg Attorney-Brumbaugh, Graves, Donohue & Raymond [57] ABSTRACT Small electric resistance elements for temperatures in the range of up to 1,680 C and made of at least 70 percent by volume MoSi, and up to 30 percent by volume of an addition having grain growth inhibiting properties at high temperatures, the particle size in the body of the finally manufactured element being less than one-fifteenth of the cross dimension of the element in the incandescent zone thereof and, in any case, less than 20 microns and further provided with a dense superficial layer of SiO, glass the thickness of which is between 1 and 8 microns. The incandescent zone suitably has a rectangular cross section having specified dimensions. The grain growth inhibiting addition may suitably be silicon carbide or a glass.

7 Clains, 1 Drawing Figure PKTENTEDM 1 I 72 INVENTORS. BENGT MAGNUSSON 8 VENANZIO BIZZARRI BY B k l G/u DMQ- their ATTORNEYS ELECTRIC RESISTANCE HEATER The present invention relates to electric resistance elements for incandescent plugs, instruments, household apparatus and similar applications.

As a rule such elements are built up of a thin metal wire, commonly of a nickel-chromium alloy or an ironchromium-alumium-cobalt alloy. Elements of this type have obtained a wide application and withstand a maximum temperature of the wire of 900 l,l C depending on the wire diameter.

For certain purposes it has been found desirable to use resistance elements at a still higher temperature and for this purpose it has, inter alia, been proposed to use elements of molybdenum disilicide. This substance is in general, however, a very brittle material, particularly after use at a high temperature, which is due to the growth of the grains in the substance. At a diameter of 3 to 2 millimeters or less rods manufactured of molybdenum disilicide by powder metallurgical techniques are so brittle and so difficult to manufacture that hitherto it has not been possible to find any practical use for them. There also is a pronounced risk of corrosion in such wires. This corrosion is of two kinds, i.e. firstly, the so-called pest taking place within a temperature interval of 300 to 700 C and secondly, oxidation at high temperature producing a glass of SiO;.

The present invention has as its object overcoming at least in some degree these and other problems and provides an electric resistance element comprising wire of 0.15 to l.l5 millimeters diameter, preferably 0.35 to 0.85 mm, which is formed into a flat or cylindrical spiral, a screw thread, loops or other form, and which is manufactured in a powder metallurgical way by extrusion and subsequent sintering and consists of at least 70 percent by volume of MoSi and up to 30 percent by volume of an addition counteracting grain growth at high temperature, said addition being preferably silicon carbide or glass, the particle size in the finally manufactured wire being less than one-fifteenth of the diameter of the wire and in all cases less than 20 microns. The relation between the length of the incandescent zone of the wire and its diameter should preferably exceed 200.

It is particularly important that the wire is surrounded by a dense coating of SiO glass which should be practically free from fissures and, on the one hand, sufficiently thin to adhere to the wire but, on the other, sufiiciently thick to offer a satisfactory protection for the wire against oxidation attacks. These conditions mean that the thickness of the layer is critical and can vary only between 1 and 8 microns. The most suitable thickness has been found to fall between 2 and 5 microns.

The grain growth inhibiting addition should be selected taking into consideration that it should be active both within a moderate temperature interval, such as from 300 up to 700 C, and within a high temperature range, such as from 1,400" up to l,680 C.

In its glowing state such a wire is plastically deforma ble but also after use and in cold condition the wire will be sufficiently elastically yielding as to be able to take up stresses due to temperature variations and the like.

According to the condition set forth above a diameter of 0.15 mm corresponds to a grain size not exceeding one-fifteenth thereof or 0.01 mm or 10 microns. A diameter of 0.30 to 1.l5 mm corresponds to a highest grain size of microns. These statements should be so interpreted that at most about l5 percent, preferably 5 percent, by weight, of the material could have a larger grain size.

The basis of the present invention is the surprising observation that a wire or a narrow rod composed in the manner stated above offers a combination of properties which makes it suitable for the manufacture of incandescent plugs, instruments, household apparatus etc., i.e. that in cold condition and within the stated range of cross sections the wire is elastically yielding without bursting but that the same wire when heated is plastically deformable and that the grain growth inhibiting addition added in a powder metallurgical way has the result that after heating to a high temperature, both in the sintering during manufacture, and in practical use, the material in the wire will retain substantially its small grain size and not be brittle to such an extent that the elastic properties in cold condition are lost. Through this combination of properties the element according to the invention will be useful in practice for its purpose.

In cold condition and when newly manufactured and at a temperature of +20 C the wire affords a bending breaking strength of at least kg/mm and generally more than kg/mm.

A wire of this nature may be tested by a shock or fatigue treatment. Then electric current is brought to pass through the wire surrounded by air for 10 seconds whereby it is rapidly heated to 1,400" C whereupon the current is disconnected and the wire is allowed to cool off for 5 seconds. Such a heating and cooling cycle is repeated 100,000 times. The break bending strength of the wire is reduced through this treatment at most about 40 percent, such as from 60 to 36 kg/mm.

The element according to this invention may have the shape of a cylindrical spiral, which rests entirely or partially against the inside of a tube of a fireproof material, preferably transparent quartz glass, in such a way that the loops of the element have a direct contact with the inside of the tube only at a small number of points. As the quartz glass crystallizes only at l,000 up to l,l00 C the advantage is gained that this glass is heated above this temperature limit only in a few points, i.e. where the resistance element of molybdenum disilicide is in direct contact with the tube wall. At all other places the quartz glass tube lets through the radiation from the element without being itself heated above the critical temperature.

As an alternative the element may have the shape of a flat spiral resting loosely against an even fireproof support, such as a brick of sillimanite.

Practical tests have shown that the elements endure a heating effect in air of the order of 80 up to W/cm of the surface of the wire and that also after a pro longed use at temperatures exceeding l,500 C the wire is still elastic and comparatively strong in the cold state. In comparison with earlier known elements containing ferritic alloys of the type iron-chromium-aluminumcobalt (trade mark KANTHAL) the elements according to the present invention have the feature that they are non-magnetic whereby the inconveniencies caused by radio interference and so called humming are eliminated. The highly positive temperature coefficient of the electrical resistance causes the elements to quickly reach a high temperature when the current is put on. Variations in the temperature along the incandescent zone are also eliminated to a high degree through the influence of the positive temperature coefficient.

An element according to the invention having a wire diameter of about 0.4 mm and a sufficient length may be connected directly to the electric mains without use of transformers. At a surface load of 60 W/cm the current will reach a value of 6 amperes. Due to the low cold resistance of the wire there is a heavy current pulse when the element is first connected in circuit but any inconveniencies caused thereby may easily be eliminated by controlling the element with a thyristor; thyristors are obtainable commercially at a comparatively low price.

EXAMPLE A wire had the composition 80 percent by volume MoSiand 20 percent by volume glass and a diameter of 0.50 mm. It was manufactured by sintering in two steps from an extruded mass containing 95 percent by weight MoSi, of a grain size less than 10 microns and percent by weight of a glass-forming clay, preferably bentonite. The first sintering step was made in hydrogen gas at l,300 C for a time of 30 minutes and the second step was made in air at 1,600" C for a time of seconds. The air-sintered wire showed under microscopic examination a glass coating of a thickness of between 3 and 4 microns. The grains were of varying sizes and more than 50 percent thereof had an approximate diameter of 8 microns or less. A few grains, less than 5 percent, were, however, discernable having a diameter of up to microns.

At +20 C it was found that the specific resistance was 0.315 ohm mm lm. At +1 ,500" C this increased to 3.41 ohm mm /m. At +20 C the bending break strength was 67 kglmm After fatigue tests according to the above this strength was reduced to 43 kg/mm? After heating to 400 500 C for 1,000 hours only insignificant traces, if any, of molybdenum silicide pest at the outer surface of the wire could be discovered.

Such a thin wire element may be used in ignition of gas, such as in a burner for cooking purposes or in a central heating boiler, the wire having a diameter of 0.4 mm and being soldered on to metallic conductors. Such elements have been found to operate satisfactorily in the ignition of gas in which case the incandescent zone operates at a temperature of about L400 C. An incon' venience is that the wire is brittle and difficult to manufacture on a large scale at a low price. Sometimes difficulty arises in the ignition of certain gas compositions so that a very high element temperature must be used. It has been found that an important reason for this is that round wire does not produce any turbulence in the gas to be ignited. To avoid deterioration of the wire at an elevated temperature it has been proposed to use several turns of the wire forming a spiral to produce turbulence and thereby render possible the use of a more moderate temperature. It is, however, still more difficult to produce wires of this type shaped as spirals on an industrial scale and the tendency of the molybdenum disilicide to soften at a high temperature is a pronounced disadvantage.

The present invention is of particular interest for electrical resistance elements which are used in the ignition of gas or oil; the invention comprises an incandescent zone adapted for operating temperatures in the range of 1,100 to 1,700 C and cold terminal zones attached thereto and which consists substantially of molybdenum disilicide. An important feature of the invention is that the conducting area of the incandescent zone is rectangular and 0.3 up to 4.0 mm, each side of the rectangle being at least 0.5 mm.

The feature that the incandescent zone has a rectangular cross section will produce turbulence when the gas to be ignited is passing the corners. The proposed shape of the element makes it easy to manufacture on a large industrial scale in automatic machinery and without the necessity of any soldering.

Such an element is preferably manufactured as follows. A pulverulent mass is introduced in a press, then subjected to compression and thereupon sintered. The powder used in filling the press mould has preferably a grain size of 0.05 0.75 mm, each grain being a conglomerate built up of a great number of particles of MoSi of grain size I to 10 microns. Said individual particles of molybdenum silicide are preferably held together by means of a binding means, such as bentonite.

EXAMPLE A pulverulent mass is manufactured by drying a humid mass containing 95 percent by weight MoSi with a grain size of about 8 microns and 5 percent by weight bentonite and from this mass a fraction between 0.1 and 0.7 mm is screened out. The powder is filled into a mould and pressed at a pressure of 5,000 kp/cm The pressed and shaped body has then obtained the general appearance as seen from the attached drawing and has considerably larger dimensions than the resistance body in its final state. This preliminary body is then subjected to two sintering operations, the first one taking place in hydrogen gas at l,300 C and the second one in air at 1,600 C.

After this operation the element is ready to be used and the cold zones may in known manner be provided with a metal coating, such as by flame spraying of aluminium, to facilitate their connection to terminal contacts. An element made according to the invention will now be described with reference to the accompanying drawing showing a final element body in perspective view.

The element is flat and formed as the letter omega and has an annular incandescent zone 1 with a rectangular cross section, the radial dimension a being 1.1 mm and the axial thickness b being 0.8 mm. The outer diameter D is 5.2 mm and the interior diameter d 3.0 mm. The ends of the part 1 merge into two parallel branches 2 and 3 and these merge in turn into the terminal cold zones 4 and 5. Between the branches 2 and 3 and the zones 4 and 5 there is a narrow air gap 6. At a temperature of l,3 50 C this element operated with 23 amperes and 1.9 volts and could be used for igniting propane in air.

What we claim is:

1. Small electric resistance elements for temperatures in the range of up to l,680 C and made of at least percent by volume MoSi and up to 30 percent by volume of an addition having grain growth inhibiting properties at high temperatures, the particle size in the body of the finally manufactured element being less than oneifteenth of the cross dimension of the element in the incandescent zone thereof and less than microns and further provided with a dense superficial layer of SiO, glass. the thickness of which is between I and 8 microns.

2. The element of claim 1, characterized in that the incandescent zone has a rectangular cross section having an electrically conducting area in the range of 0.3 4.0 mm, each side of the rectangle being at least 0.5

3. The element of claim 1 in which the grain growth P040561 UNITED STATES PATENT OFFICE CETTETEATE l EEETAN Patent NO. 3,6 1,737 Dated August 1, 1972 Inventor) Bengt Magnusson and Venanzio Bizzarri It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Page 1 and Column 1, the title should be changed from "ELECTRIC RESTANCE HEATER" to ELECTRIC RESISTANCE ELEMENTS Page 1 Column 1, line 8, change "25,731/69" to 25, 3 9 --3 Column 5, line 2, change "oneifteenth" to one-fifteenth Signed and sealed this 23rd day of January 1973.

(SEAL) Attest! EDWARD M. FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents PO-WBO UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 a 737 Dated August 1 97 Inentor(S) Bengt Magnusson and Venanzio Bizzarri It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Page 1 and Column 1, the title should be changed from "ELECTRIC RES'IANCE HEATER" to ELECTRIC RESISTANCE ELEMENTS Page 1, Column 1, line 8, change "25,731/69" to 25 231/69 5 v 1 Column 5, line 2 change "oneifteenth" to one-fifteenth Signed and sealed this ZSrdday of January 1973.

(SEAL) Attest:

EDWARD M. FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 5 23 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 681 737 Dated August 1 97 Inventor s Bengt Magnusson and Venanzio Bizzarri It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Page I and Column 1, the title should be changed from "ELECTRIC RESTANCE HEATER to ELECTRIC RESISTANCE ELEMENTS Page 1, Column 1, line 8, change "25,731/69" to Column 5, line 2, change "oneifteenth" to one-fifteenth Signed and sealed this 23rd day of January 1973.

SEAL) ttest'.

DWARD M.FLETCHER,JR. ROBERT GOTTSCHALK ttesting Officer Commissioner of Patents 

2. The element of claim 1, characterized in that the incandescent zone has a rectangular cross section having an electrically conDucting area in the range of 0.3 - 4.0 mm2, each side of the rectangle being at least 0.5 mm.
 3. The element of claim 1 in which the grain growth inhibiting addition is silicon carbide.
 4. The element of claim 1 in which the grain growth inhibiting addition is a glass.
 5. The element of claim 1 which has the shape of a wire with a diameter of 0.15 to 1.15 mm.
 6. The element of claim 5, in which the diameter is in the range of 0.35 - 0.85 mm.
 7. The element of claim 5 in which the relation between the length of the wire and its diameter exceeds
 200. 